1. Field of the Invention
Embodiments of the present invention generally relate to methods and systems and, more particularly, to mechanisms and techniques for similitude testing of compressors.
2. Description of the Prior Art
A compressor is a machine which increases the pressure of a compressible fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a number of different applications. Among the various types of compressors used in such processes and process plants are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration which accelerates the gas particles, e.g., by rotating a centrifugal impeller or rotor through which the gas passes.
Centrifugal compressors can be fitted with a single impeller or stage, a single stage configuration, or with a plurality of stages in series, in which case they are frequently referred to as multistage compressors. In turn, a specific sub-family of multi-stage compressor comprises a multi-section multistage compressor which is configured such that the totality of the compressor flow is extracted from the compressor, cooled down and then re-injected into the compressor. Most of the time, the number of sections in this sub-family of multistage compressor is limited to two which sections can be arranged in either an in-line or a back-to-back configuration depending on a relative orientation of the impellers of a second section with respect to the impellers in a first section.
Each of the stages of a centrifugal compressor typically comprises an inlet conduit for gas to be compressed, an impeller or wheel which is capable of providing kinetic energy to the input gas and an exit system, referred to as a stator, which converts the kinetic energy of the gas leaving the rotor into pressure energy. Multiple stator component configurations can be used, the most common ones being the vaneless diffuser, the vaned diffuser return channel, discharge scroll or plenum or combinations of these configurations. The combination of an individual impeller and its associated stator component is typically referred to as a stage.
During the compressor design development phase and prior to being placed into service by end users, centrifugal compressors are typically subjected to a number of different types of tests as part of, for example, performance validation activities and quality control processes. One such test is a similitude test, which attempts to ensure that the centrifugal compressor will operate in the manner desired by an end user with respect to flow rate, pressure rise, efficiency and speed for a given input gas. To perform the similitude test, a manufacturer operates the centrifugal compressor under test conditions which are intended, to be as close as possible to those which will be experienced by the centrifugal compressor at the end user's client's site. To achieve these objectives, it is desirable to provide similitude testing systems and methods for compressors wherein the test gas used in the compressor is appropriately selected. Typically, the gas R134A (also known as HFC-134a) is internationally used in purity or mixture as working fluid to provide the similitude test.